Along with the reduction in weight and improvement in performance of automobiles, springs are being made higher in strength. High strength steel having a tensile strength exceeding 1500 MPa after heat treatment is being used for springs. In recent years, steel wire having a tensile strength exceeding 1900 MPa is also being sought. This is so as to secure a hardness of material where even with some softening due to straightening annealing, nitridation, and other heating at the time of spring production, there is no problem for the spring.
Further, it is known that with nitridation or shot peening, the surface hardness rises and the durability during spring fatigue is remarkably improved, but the spring setting characteristic is not determined by the surface hardness. The internal strength or hardness of the spring material also has a great effect. Therefore, it is important to devise ingredients able to maintain the internal hardness extremely high.
As a technique for this, there is an invention adding V, Nb, Mo, or another element, dissolving this by quenching, forming fine carbides precipitated by tempering, and thereby limiting the action of dislocation and improving the anti-setting property (for example, see Japanese Patent Publication (A) No. 57-32353).
On the other hand, among the methods for production of steel coil springs, there are hot coiling of heating the steel to the austenite region for coiling, then quenching and tempering it and cold coiling of quenching and tempering the steel in advance and cold coiling the resultant high strength steel wire. With cold coiling, it is possible to use oil tempering, high frequency treatment, etc. enabling rapid heating and rapid cooling at the time of production of the steel wire, so it is possible to reduce the prior austenite grain size of the spring material. As a result, it is possible to produce a spring superior in breakage characteristics. Further, it is possible to simplify the heating furnace and other facilities on the spring production line, so there is the advantage to the spring manufacturers as well that this leads to a reduction in the capital costs. Recently, cold coiling is being employed even for large diameter suspension springs. In this way, processes are being converted to cold coiling.
However, if the spring-use steel wire for cold coiling increases in strength, it will break at the time of cold coiling and will be unable to be formed into a spring shape in many cases. In this case, both strength and workability cannot be achieved, so the wire has to be coiled by industrially disadvantageous methods. Usually, in the case of a valve spring, steel wire quenched and tempered on-line, so-called oil tempered steel wire, is often cold coiled, but for example there are inventions heating the wire to 900 to 1050° C., coiling it to a spring shape, then tempering it to 425 to 550° C. and otherwise preventing breakage at the time of coiling by heating the wire material, hot coiling it at a temperature where deformation is easy, then thermally refining it to obtain a high strength (for example, see Japanese Patent Publication (A) No. 5-179348). Such heating at the time of coiling and thermal refining after coiling become causes of variations in spring dimensions due to heat treatment and result in a sharp drop in the processing efficiency, so the resultant springs are inferior to cold coiled springs in respect to cost, precision, and product stability.
Further, regarding carbides, for example, there are inventions focusing on the average grain size of the Nb- and V-based carbides. These show that control of the average grain size of the V- and Nb-based carbides alone is insufficient (for example, see Japanese Patent Publication (A) No. 10-251804). In this prior art, it is described that there is a concern over the formation of abnormal structures due to cooling water during rolling. In practice, dry rolling is recommended. This is industrially unstable work and is considered clearly different from the usual rolling. Even if controlling the average grain size, if the surrounding matrix structure becomes uneven, it is suggested that rolling trouble will occur.
Further, there is an invention aiming at improvement of performance by controlling the cementite and other carbides (for example, see Japanese Patent Publication (A) No. 2002-180198).
However, to further improve the fatigue, setting, and other spring performance, further higher strength and spring workability (coilability) have to be secured. There were limits with the ingredients and control of dimensions of the carbides after heat treatment up to now.
In this way, technology for achieving both strength and workability is being searched for. Achievement of both strength and workability has been sought by control of the structure focusing on the cementite-based carbides (Japanese Patent Publication (A) No. 2002-180198). Further, stability is being increased by preventing residual austenite (for example, see Japanese Patent Publication (A) No. 2000-169937). These are largely due to the heat treatment steps. On the other hand, with valve springs, oxides are mainly being controlled. Improvement of fatigue strength by control of oxides is being argued. Oxides are believed to affect not only the fatigue strength itself, but also the stability of the breakage resistance characteristic and product variations. Suppression of the rate of appearance of inclusions at the breakage faces is being sought (for example, see Japanese Patent Publication (A) No. 6-158226).
Further, if not only oxides, but also sulfides, nitrides, carbides, and their composite inclusions are present, the fatigue strength is lowered and a drop in workability is caused. In steel having an extremely high tensile strength such as for valve springs, in Patent Document 6, attempts have been made to control the TiN and further the carbides (for example, see Japanese Patent Publication (A) No. 10-251804), but few technology has considered sulfides as well.
As examples focusing on sulfides, there are ones considering the addition of at least one of Ti, Cu, Ca, and Zr to be effective, but in these examples, the majority concern addition of Ti. Even when not adding Ti, large amounts of Zr, Ca, and other oxide producing elements are added (for example, see Japanese Patent Publication (A) No. 10-1746). If considering one of the characteristic features of the present invention, Zr, since a large amount of 10 ppm or more (in the examples, 70 ppm) is added, there is a large effect on the oxides, the fatigue strength is lowered, the rate of appearance of inclusions rises, or other problems occur.
Further, as other examples, there are ones considering addition of Zr to be effective (for example, see Japanese Patent Publication (A) No. 2003-105485). A large amount of 10 ppm or more (in the examples, 23 ppm) is added, so there is a large effect on the oxides, the fatigue strength is lowered, the rate of appearance of inclusions becomes high, and other problems arise.
Further, there are inventions showing that the amount of addition of Zr should be suppressed to 0.5 ppm or less in solid solution in the steel and clearly indicating that if over this, problems arise due to inclusions (for example, see Japanese Patent Publication (A) No. 9-310145). However, with this amount of addition, control of the sulfides is insufficient. This is easily deduced from the above mentioned Patent Document 8.